1. Field of the Invention
The invention relates generally to forward converters, and in particular, to a forward converter that operates beyond the customary 50 per cent duty cycle limit.
2. Description of the Related Art
Forward converters are used to transfer input power, the product of a defined input voltage and current, to an output providing substantially the same amount of power, the product of a desired output voltage and current. Typically, forward converters consist of a transformer having a primary winding connected to a primary circuit including input terminals for connection to the input energy source, and one or more secondary windings connected to a secondary circuit including output terminals for connection to the load. The primary circuit includes a transistor switch, which in operation, controls the application of the input energy to the primary winding of a transformer, alternating between an ON period and an OFF period, at a predetermined rate and duration. "Buck converters" are forward converters which transfer power from the primary circuit to the secondary circuit during the ON period.
At each ON period, current flows from the input energy source through the primary winding of the transformer, creating a magnetizing inductance in the core of the transformer. As a result, during the OFF period, parasitic energy is present in the transformer resulting from leakage inductance and magnetizing inductance. This parasitic energy must be removed from the transformer before the subsequent ON period.
Until now, the parasitic energy was removed from the transformer by employing snubbers, energy recovery networks and demagnetizing windings. However, these means limit the maximum duty cycle of forward converters to about 50 per cent.
It would be advantageous to produce forward converters with duty cycles up to 90 per cent, that operate over a wider range of input voltages, uses simpler components which would increase reliability and reduce power loss.